This invention relates to a new process for dewatering municipal and other sewage sludges containing fat-type and proteinic organic contaminants, and for separating, respectively, the solid particles of such sludges from the aqueous phase.
At present, the sewage arising mostly from households, from commercial kitchens as well as from the milk industry and other plants of the food industry, which contain mainly carbohydrates, fats and proteins as organic contaminations, are usually treated in such a way that the sewage led through a coarse lattice-type filter is first introduced to a grinding or stamp mill, then the lumpy and in general inorganic solid materials are separated by leading through a so-called sand-trapping settler in order to purify the sewage and optionally to utilize the organic solid particles. Then, the thus pre-treated sewage is subjected to a biological decomposition in an aerating reactor under aerobic conditions. This process can usually be accomplished within a few (6 to 12) hours when the adequate temperature and oxygen amount is provided. Subsequently, two routes of purifying can be used. According to one of the common methods, the sludge is led into a settling tank, where as coagulating agents iron chloride, iron sulphate, aluminium sulphate and other inorganic chemicals, such as caustic lime are added for preparing the dewatering of the sludge in vacuum filters and filter presses. Under the effect of these agents, the settled slurry may further be dewatered on appropriate filters, while the water may be discharged.
According to the other common method, after having been decomposed biologically in the aerating reactors, the sludge is led into a settling tank, where the solid particles are settled, the water made free of those is discharged and the slurry obtained in the settling tanks is led into closed, large-sized vessels, usually of several thousands cubic meter volume, where it is digested (decomposed) with the exclusion of atmospheric oxygen, optionally under an inert gas, under anaerobic conditions at 30.degree. to 35.degree. C. for a longer time, e.g. for 30 days, or at 50.degree. to 52.degree. C. for a shorter period, e.g. for 10 to 12 days, that is the sludge is subjected to a further, anaerobic biological decomposition. During the anaerobic digesting (decomposition) a methane-containing gas, the so-called biogas is evolved which may be utilized on one part for maintaining the temperature of the digesting system and on the other part eventually for other purposes.
In the case of a discontinuous plant operation, one cycle of the anaerobic decomposition (putrescence) proceeds within about 30 days. However, under plant conditions the process is usually accomplished halfperiodically, in a continuous cycle in such a way that an amount of the putrefied sludge adequate to a defined part of the reactor volume (e.g. 1/30) is daily taken off from the large-sized digesting reactor and the same volume of a fresh sludge is fed into the reactor. The dry substance content of the digested sludge taken off is generally between 3% and 6%, usually about 4%. This sludge is treated with flocculating agents, then led onto filters or other dewatering apparatuses and dewatered to the greatest possible extent.
The up-to-date methods described above for treating sewage sludges are summarized e.g. in the book of I. S. Turovski titled "The Treatment of Sewage Sludges" [Musazaki Konyvkiado (Technical Publishing House), Budapest, 1980].
The most critical step in the processes, mentioned above for working up sludges, is the dewatering which can only be realized by using power-demanding mechanical equipment and a high time-input with a low efficiency. Thus, the water content of the filter-cake can only be lowered to 75 to 80% by using band filters with possibly slow operation, that is, with a long cycle-time, while by using a filter press the water content cannot be reduced to a level lower than 65%, even with a filter performance of 5 to 6 kg/m.sup.2.hour.